A variety of systems have been developed for filtering water and/or wastewater. Typical filter systems include but are not limited to an upflow filter, a downflow filter, a combined upflow filter and downflow filter and multiple downflow filters connected in series. The term upflow filter is given to a filter in which the liquid or influent to be filtered is directed in an upward path to remove impurities. Conversely, a downflow filter is a filter in which the influent is directed in a downward path to remove impurities.
In a combined upflow/downflow filter system, influent is directed upwardly through the upflow filter to remove a predetermined percentage of the impurities in the influent and then the influent is directed downwardly through the downflow filter to remove the remaining impurities to within an acceptable level. The upflow filter, in this combined system, is referred to as a roughing filter or clarifier. The downflow filter, in this combined system, is referred to as a polishing filter. One noticeable difference between a roughing filter and a polishing filter is the size of the filter media. The filter media in the polishing filter is considerably smaller than the filter media in the roughing filter.
The most common methodology utilized to specify the size of media in the filtration industry is through effective size and uniformity coefficient. The American Water Works Association (AWWA) B100 standard defines effective size (also known as d10) as “the size of opening that will just pass 10 percent (by dry weight) of a representative sample of filter material; that is, if the size distribution of particles is such that 10 percent (by dry weight) of a sample is finer than 0.45 mm, the filter material has an effective size of 0.45 mm. ” As used herein “effective size” has the same meaning as the AWWA B100 standard.
The AWWA B100 standard defines uniformity coefficient as “a ratio calculated as the size opening that will just pass 60 percent (by dry weight) of a representative sample of the filter material divided by the size opening that will just pass 10 percent (by dry weight) of the same sample.” As used herein “uniformity coefficient” has the same meaning as the AWWA B100 standard.
A typical specification for filter sand used in a polishing filter is an effective size ranging from 0.45 mm to 0.55 mm with a uniformity coefficient of less than 1.7. The effective size of the filter sand used in a roughing filter is considerably larger and can have an effective size well in excess of 1.0 mm. The smaller particles in the filter sand used in polishing filters can lead to clogging of the underdrain. For example, one common type of underdrain includes a plurality of underdrain blocks arranged in parallel rows across the bottom of the filter. The underdrain blocks act to direct and receive fluids including influent, effluent and air during operation of the filter system. The underdrain blocks typically include multiple apertures through which the fluids are directed and received. The apertures are typically larger than the smaller particles of the filter sand used in polishing filters. Accordingly, it has been necessary to employ some means to prevent clogging and/or structural failure of the underdrain.
One or more gravel support layers have been used between the filter sand and the underdrain to prevent clogging. Referring to FIG. 1, a prior art filter is depicted using a gravel support layer 2 between the underdrain blocks 4 and the filter sand 6 to prevent clogging of the underdrain blocks 4. The gravel in the support layer 2 is larger than the apertures in the underdrain blocks and, therefore, does not pass there through. The smaller particles of the filter sand will embed in the support gravel rather than pass through or obstruct the apertures in the underdrain blocks. However, gravel support layers have a number of disadvantages. Specifically, gravel support layers are expensive and time consuming to install. Further, gravel support layers consume a significant portion of the filter chamber thus reducing the filtering capacity of the bed. Also, gravel support layers are subject to being upset when uncontrolled air enters the filter bed due to installation of the air system or operator error. Moreover, in filter beds using granular activated carbon such must occasionally be removed from the filter and placed in a reactivation furnace. During removal of the granular activated carbon, the gravel becomes intermixed and is deposited in the reactivation furnace. At the extreme temperatures necessary to reactivate the granular activated carbon the gravel can explode damaging the furnace.
To overcome the disadvantages of gravel support layers, porous plates have been used with underdrain blocks. The porous plates obviate the need for the gravel support layers because they prevent the filter media from passing through or lodging in the apertures in the underdrain blocks. Referring to FIG. 2, a prior art filter is depicted using a porous plate 8 between the underdrain blocks 10 and the filter sand layer 12. Porous plates are typically formed from sintering plastic beads such as high-density polyethylene into an open-celled structure. The porous plates typically have a thickness ranging from ¾″ to 1″ and have an average pore size of 300 to 700 microns.
Porous plates are typically attached to the upper surface of an underdrain block with screws, a rails system or other attachment means. In most cases, the filter sand having an effective size ranging from 0.3 mm to 0.5 mm is placed directly on top of the porous plate. The fine particles in the filter sand will nest in the pores of the porous plate and eventually pass through the porous plate. The presence of the fine filter media particles embedded in the pores of the porous plate can accelerate clogging and lead to structural failure of the plate. A two or three inch layer of torpedo sand having an effective size ranging from 0.8 mm to 1.2 mm has been used between the porous plate and the filter sand layer to prevent the fine particles from embedding in the porous plate. However, the torpedo sand suffers from problems similar to those associated with one or more gravel support layers. Further, members in the water filtration community are reluctant to add additional layers of media, substitute a layer of torpedo sand for the corresponding depth of filter media or otherwise alter the filter media specifications.
Accordingly, there is a present need for a filter that does not use either torpedo sand or gravel and yet still prevents clogging and/or structural failure of the porous plate or other gravel-less underdrain.